Flight Altitude Restriction and Supercruise

The F-22 can supercruise at a wide variety of altitudes, including at extreme high altitudes. The speed may change slightly, due to the effects of altitude on Mach numbers.

The F-22 has improved high-altitude performance because of a number of factors. Engine airflow, intake design & wing loading. Some reports have the pilots using the afterburners above 50,000 (alt Restrictions not withstanding) but other reports say that once at altitude, the F-22 will supercruise quite fast at 60,000. The F-22 has significantly less drag on the airframe, compared to other jets like the F-15.

The F-22 can supercruise at M1.82 and reach an altitude of over 50,000 ft, but not at the same time. Actually, best supercruise performance is supposedly at 35,000 ft.

Agreed, the jet engines lose thrust as altitude increases. The optimum altitude for cruise is somewhere between max thrust and max altitude. Normally 30k-40k feet

Doesn't the Mach number of a particular airspeed also increase as you go up in altitude?

Yes. Assuming the ideal gas model and that the gas is calorically perfect (in that the ratio of the specific heat of air at constant pressure to the specific heat at constant volume is constant) then the speed of sound will decrease as temperature drops. At high altitudes the temperature is lower so a given airspeed will yield a higher mach number.